DOCS: Red/System specification reference updated to revision 42.
FEAT: compiler's options base object moved to beginning of file (improves readability).
FEAT: Added `pi` word definition.
FEAT: added new compilation option: ABI (for selecting a specific ABI when needed).
FEAT: added new RPi compilation target.
FEAT: hard-float (armhf) ARM support added.

Set the right ABI for ARM targets in config.r when needed:
    ABI: 'hard-float
will set the armhf mode instead of the default armel.
FEAT: added mention for RPi new target in README.
FEAT: added default `pi` definition for Red/System.
TESTS: added RPi target option to ARM tests building scripts.
TESTS: fixes in float32 tests to avoid collision with predefined `pi` value.
FEAT: armhf support extended to syscalls and imported functions pointers.
FEAT: Added armhf support for Red/System callbacks.

Warning: untested code atm.
FIX: callbacks arguments are now pushed in right order on ARM armhf.
FIX: change linker path for ARM hard-float Linux.

FIX: remove unnecessary PI definition in TESTS.
Merge pull request #888 from qtxie/armhf-linker

FIX: change linker path for ARM hard-float Linux.
FIX: use %.16g for printing float! value.
FIX: compiler can't keep float! literal's precision

FIX: improve accuracy for float! to integer! conversion.
TESTS: add tests for checking float! literal's precision.
Merge pull request #889 from qtxie/print-float

FIX: use %.16g for printing float! value.
FEAT: added radians-oriented trigonometric functions: sin, cos, tan, arcsin, arccos, arctan.
FEAT: added missing `routine?` function.